GB919967A - Improvements in or relating to arrangements embodying pick-up tubes - Google Patents

Abstract

919,967. Television pick-up tubes. ELECTRIC & MUSICAL INDUSTRIES Ltd. June 5, 1959 [June 18, 1958], No. 19437/58. Class 39 (1). [Also in Group XL (b)] The effect of " lag " in the target 2 (Fig. 1) of a photo-conductive type pick-up tube is reduced by imaging the scene on to the target via an optical strip-filter 3 alternate strips of which are fully transparent and intervening strips semi-transparent so that on scanning the target transversely of the strips (or along the strips if a 4 Mc/s. " spot-wobble " is utilized) by means of an electron beam a modulation component due to the strips which is of amplitude proportional to " lag " is produced and this component is employed to reduce the " lag " component in the video output signals from the tube. Fig. 2 shows the strip modulation 5 for a constant white light of average level 6 and the strip modulation 7 for constant grey light of average level 8. From this it is seen that the amplitude of the strip modulation is proportional to the average level of a constant light stimulus. If, however, the white light is suddenly removed from the target the " lag " in the latter prevents the output signal from immediately falling to black level and after one or two frames a level equivalent only to that shown at 8 in Fig. 3 (about the same as the grey level 8 in Fig. 2) will have been reached. The amplitude of the strip modulation component in this case is however much less than that shown at 7 (Fig. 2) and by comparing this modulation with the average value of the video signal in a suitable computer all " lag " signals may be suppressed and " true " video signals only, corrected if necessary for the effect of "lag" allowed to pass. The strip filter may comprise 650 elements (325 transparent and 325 semitransparent) giving rise to a modulation at about 4 Mc/s. so that (as shown in Fig. 4) the combined video and strip modulation signals in the output of an amplifier 10 supplied by the tube 9 may be separated by means of a highpass filter 11 passing the modulation component and a low-pass filter 12 passing video signals up to about 3 Mc/s. These two signals are then supplied to a circuit 13 in which " true " video signals are recognized by the simultaneous occurrence of a modulation component having an amplitude equal to that which is produced by a constant intensity light signal of the same average level as the video signals considered. Such signals are then " passed-on " to the video amplifier 14. If the amplitude of the modulation component is very much less than that produced by a constant intensity signal of the level of the particular video signals considered the latter are recognized as due to the " lag " effect of the target and are prevented from being " passed-on." When the amplitude of the modulation component has a value intermediate that expected of " true " and " lag " video signals the particular video signal is recognized as a " true " signal resulting from a change in the intensity of the light stimulus on the target and in such cases the circuit 13 operates to subtract the " lag " portion and " pass-on " a corrected signal to amplifier 14. In addition to compensating for such " lag " (delay lag) effects the circuit 13 may also be arranged to provide compensation for build-up "lag" effects. Instead of employing the optical strip-filter 3 (Fig. 1) the target electrode may comprise strips of photo-conductive material alternate ones of which have a lower sensitivity than the intervening ones. When employing the optical strip-filter with transverse scanning an improvement in the strip modulation is achieved by reducing the total number of strips to 200 (instead of 650). It is then necessary to prevent the strip structure from being visible in the reproduced picture and this is effected by pulsing the tube cathode in such manner that, for a constant intensity light image, no strip modulation is produced. A strip modulation is, however, produced when the light intensity changes, due to the " lag " effect and this is utilized in the manner described above. To synchronize the pulsing of the cathode with the position of the scanning beam relatively to the strips suitable indexing signals are produced by the signal electrode by making the latter in the form of two sets of intermeshing fingers, each finger being associated with a single strip (Fig. 5, not shown). Specification 784,863 is referred to.